Originally published in Global Water Forum, Monday 3 October 2016. To maximise downstream water quantity, you remove vegetation – all of it, including the trees. To counter rising carbon dioxide levels, you plant trees – lots of them. How should we do both?
Written by Dr. James Dalton, International Union for Conservation of Nature, Switzerland; Dr. David Ellison, Senior Researcher, Ellison Consulting, Switzerland; Dr. Matthew McCartney, International Water Management Institute, Laos; Dr. Jamie Pittock, Associate Professor, the Australia National University, Australia; and, Brendan Smith, Gold Standard Foundation, Switzerland
Inaccurate assumptions about forest-water interactions are persistent, pernicious, and can lead to poor management and policy decisions. How forests affect climate and hydrology is complex. The effects of removing or adding forest cover on water resources vary with scale, spatial organisation, and management practices. More forests do not necessarily lead to more water.
Trees are significant “consumers” of water. In some arid parts of the world, the removal of trees is a strategy to increase river flow. However, we need trees for the multiple benefits and services they provide, such as the habitats they create, the carbon they store,1 and the water services they supply.
Reforestation and afforestation are some of the main climate change mitigation strategies being used to counteract rising carbon dioxide levels in the atmosphere. For example, the UN-REDD+ programme focuses on reducing forest emissions and enhancing carbon stocks in forests while contributing to sustainable development. Yet, mitigation actions such as those promoted by REDD+ should be implemented with consideration of the water consequences of re/afforestation. Water is a fundamental resource that trees, in the right circumstances, can help sustain. Yet planting trees for carbon storage alone may in some circumstances diminish water resources.
When planning re/afforestation activities, where is it best to plant trees that bring multiple benefits for carbon storage, other ecosystem services, and water? Are our land, water, and mitigation strategies integrated to maximise the benefits?
It remains unclear how strategies designed to restore landscapes assess impacts on water. Restoration actions often claim benefits that are not monitored and verified or may take many years to realise—years when other activities in river basins may offset or confound the intended water and carbon benefits. The length and scale of landscape change make it difficult to identify the response of the overall forest and water ecosystem, which in turn inhibits learning about which restoration actions generate the best outcomes.
Careful attention must therefore be paid to maximising the benefits while minimising the impacts on downstream water supplies. Several generations of paired-catchment basin studies have investigated the links between reforestation and catchment runoff.1,2 The results highlight the complexity of the processes, for example in Australia one modelled study predicted that reforestation of just 10% of one river’s headwaters would diminish water flows in an already over-allocated river basin by 17%.3 Land and water management practices hence play a significant role in how a catchment responds to changes in forest cover. Scale and spatial organisation are important.
Hydrology focuses on river basins as the unit of assessment but has to also maintain a link to the wider climate because most precipitation is not generated from within the same river basin. Recent research shows that globally up to 40% of rainfall originates as upwind land evaporation.4 The other 60% comes from the oceans. Rather than “consumers,” trees (and other vegetation) are perhaps best viewed as water “recyclers,”5 and thereby make important contributions to cross-continental hydrologic cycles.6
The Congo Basin is a major source of rainfall for the Sahel region;7 70% of the rainfall for the Rio de la Plata Basin in Uruguay and Argentina originates as evaporation from the Amazon forest,8 the Gulf of Guinea and moisture from across Central Africa plays an important role in generating flows for the Nile via the Ethiopian Highlands.9 Recent research also suggests that moderate tree cover can increase groundwater recharge in the seasonally dry tropics in West Africa.10 Land-use decisions in one place may therefore have serious consequences for people, the economy, and the environment in distant locations.
Yet for forests to recycle water and make it available as rainfall downwind, they must remove significant quantities of water from upstream river basins. Consequently:
- removing trees may affect rainfall (and therefore water availability) in downwind locations, and
- additional tree cover through afforestation may reduce the availability of water downstream – thereby reducing streamflow.
Because they evapo-transpire less annually, grasslands and shrubs as restoration solutions have been found to bring greater improvements in soil moisture storage and soil conservation than re-forestation in the Loess Plateau in China.11,12 However, seasonal differences abound, and solutions must be contextually applied.
At what scale therefore should forest and water activities focus in order to achieve benefits? Many forest restoration, plantation, and carbon driven interventions are focused on small scale activities. Although these bring many different benefits to our mosaic landscapes, it is not clear if they are helping from a broader hydrological perspective. We may be improving carbon storage, but in 20 years’ time we may have inadvertently squeezed some of our river basins dry by restricting water flow downstream and/or planting forest in the wrong place.
We should plant trees for the multitude of water-related ecosystem services they provide: water quality benefits, soil retention, land surface cooling, soil salinity management, physical barrier and riparian protection, freshwater biodiversity benefits, infiltration and groundwater recharge, and the fact that they can contribute to making it rain.
But when we plant trees as part of broader restoration strategies, in particular those driven by climate change mitigation investments, the focus must be on:
- recognising existing conditions, including options for wider restoration opportunities for broader benefits than carbon alone. Re/afforestation for carbon storage should not be at the expense of generating more immediate water problems.
- supporting rainfall generation through up and downwind connections while at the same time considering their impact on downstream water quantity. Upwind coastal areas are frequently the best place for re/afforestation projects to start as moisture moves from over the sea onto land. But these need to gradually be extended further inland, taking advantage of areas with abundant water resources.13
- protecting and promoting cloud and montane forests where they help encourage increased infiltration and runoff, or generate additional rain and snowfall at altitude.14,15,16
- being cautious in temperate areas where evapotranspiration from trees may significantly diminish downstream surface water flows, often of high social, economic, and biological importance. Consider re/afforestation in relevant semi-arid areas where surface run-off can be used with minimal downstream conflicts, and the wet-dry tropics where the demand for water does not conflict with other users downstream.17
- the time scale of re/afforestation activities. As precipitation is likely to change with climate change, will there be enough water for forests in the future, and how much impact are forests likely to have on water resources 20-30 years from now? How might increasing forest cover benefit downwind communities and economic activities under future climate change scenarios, and therefore reinforce adaptation strategies?
Forest management alone has neither the mandate nor the focus to manage water resources. Third-party certification has only just begun to grapple with the challenges of water management for forests. Despite often sitting between a combination of commercial interests and conservation, forest management must engage better with water managers if policy responses are to protect and harness the myriad of ecosystem services provided by forests.
Forest and water managers have to improve their understanding of the complex hydrology-forest-climate interactions and work together to create more insightful policies and programs. Water governance reform, efficiency improvements, and better allocation of water must go hand-in-hand with forest activities to optimize the management of both. Recent work in the Amazon suggests that drought temporarily shut down the Amazon Basin as a carbon sink in 2010,18 affecting the forests’ growth rate and ability to store carbon, and perhaps therefore their ability to provide downwind rainfall—known locally as ‘flying rivers.’ Deforestation may also be driving a decline in rainfall, leading to drought.19
© Rafal Cichawa
What is clear is that we should challenge many of the assumptions around forests and water through better collaborative science. Attention is needed to fill knowledge gaps and develop better tools to assess the impacts of re/afforestation and other restoration activities on downstream water flows and up and downwind precipitation engines. Assessment and improved activities instead of disciplinary assumptions are needed. We must integrate re/afforestation and wider land management activities in a coordinated manner to support the water needs of both mitigation and adaptation objectives.
Land system architecture (i.e., how we ‘plan’ and design our landscapes and river basins) will determine the success of mitigation and adaptation efforts.20 The opportunity now is for strategic planning for forest landscape restoration and/or afforestation that is adapted to the requirements of specific river basins. These must be developed with broader integrated landscape interactions in mind (from regional to continental) to avoid adversely affecting water resources. Re-visiting mitigation and landscape design policies is urgently needed to ensure we balance low-carbon with resilient development. Appropriate re/afforestation needs to be better linked to our hydrological future under 1.5oC warming. Increased collaboration amongst the science and policy communities for water and forests is needed.
A coordinated effort is needed, one that builds on existing incentives not only to keep our existing forests standing, a key objective of programmes like REDD+, but that also seeks to reforest areas where most appropriate. Central to this goal must be the recognition of water as a resource that trees both require and provide.
- Jackson, R.B., Jobbagy, E.G., Avissar, R., Roy, S.B., Barrett, D.J., Cook, C.W., Farley, K.A., le Maitre, D.C. McCarl, B.A., and B.C. Murray (2005) Trading Water for Carbon with Biological Carbon Sequestration. Science, 310.
- Trabucco A, Zomer RJ, Bossio DA, van Straaten O, Verchot LV. (2008) Climate change mitigation through afforestation/reforestation: a global analysis of hydrologic impacts with four case studies. Agriculture, Ecosystems and Environment;126:81–97.
- Herron, N., Davis, R., and R. Jones (2002) The effects of large-scale afforestation and climate change on water allocation in the Macquarie River catchment, NSW, Australia. Journal of Environmental Management, 65: 369-381.
- Keys, P.W, Wang-Erlandsson L., and L.J. Gordon (2016) Revealing Invisible Water: Moisture Recycling as an Ecosystem Service. PLoS ONE 11(3) e0151993. doi:10.1371/journal.pone.0151993
- Aragão, L.E.O.C. (2012) The rainforest’s water pump. Nature, 489, 217-218.
- Ellison, D., Futter, M., and K. Bishop (2012) “On the Forest Cover – Water Yield Debate: From Demand to Supply-Side Thinking”, Global Change Biology, 18, 806-820.
- van der Ent, R.J., Savenije, H.H.G., Schaefli, B., and S. C. Steele-Dunne (2010) Origin and fate of atmospheric moisture over continents. Water Resources Research, 49, W09525.
- Viste, E. and Sorteberg, A. (2013), The effect of moisture transport variability on Ethiopian summer precipitation.J. Climatol., 33: 3106–3123. doi:10.1002/joc.3566.
- Nobre, A.D. (2014) The Future Climate of Amazonia: scientific assessment report. Articulación Regional Amazónica, Instituto Nacional de Pesquisas Espaciais, Sao Paolo, Brazil.
- Ilstedt, U., Bargués Tobella, A., Bazie, H.R., Bayala, J., Verbeeten, E., Nyberg, G., Sanou, J., Benegas, L., Murdiyarso, D., Laudon, H., Sheil, D., and A. Malmer (2016) Intermediate tree cover can maximize groundwater recharge in the seasonally dry tropics. Scientific Reports, 6:21930.
- Chen, L., Wang, J., Wei, W., Fu, B., and W. Dongping (2010) Effects of landscape restoration on soil water storage and water use in the Loess Plateau Region, China. Forest Ecology and Management, 259, 1291-1298.
- Zhang, L., Podlasly, C., Feger, Karl-Heinz, Wang, Y., and K. Schärzel (2015) Different land management measures and climate change impacts on the runoff – A simple empirical method derived in a mesoscale catchment in the Loess Plateau. Journal of Arid Environments, 120, 42-50.
- Makarieva, A.M., Gorshkov, V.G., Sheil, D., Nobre, A.D., Bunyard, P., and B.-L. Li (2014) Why Does Air Passage over Forest Yield More Rain? Examining the Coupling between Rainfall, Pressure, and Atmospheric Moisture Content. Journal of Hydrometeorology, 15, 411-426.
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- Saénz, L., and M. Mulligan (2013) The role of Cloud Affected Forests (CAFs) on water inputs to dams.Ecosystem Services, http://dx.doi.org/10.1016/j.ecoser.2013.02.005
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- Nobre, A.D. (2014) The Future Climate of Amazonia: scientific assessment report. Articulación Regional Amazónica, Instituto Nacional de Pesquisas Espaciais, Sao Paolo, Brazil.
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James Dalton is the Coordinator of Global Initiatives in the IUCN Global Water Programme. Prior to joining IUCN in 2009 he was the Integrated Water Resource Management Adviser based at the Pacific Islands Applied Geoscience Commission (SOPAC) in Fiji. He is an irrigation engineer and holds a PhD in Civil and Environmental Engineering focussing on groundwater management in the Aral Sea Basin.
David Ellison works as a Senior Consultant, primarily in collaboration with the Swedish University of Agricultural Sciences (SLU, Umea) and on an independent basis as a Senior Researcher (Ellison Consulting). His work focuses broadly on the science, politics and policy of climate, in particular on forests and their relevance for climate and policy. David publishes broadly on carbon accounting practices in Land Use, Land Use Change and Forestry (LULUCF) in the UNFCCC and Kyoto, as well as regional and national climate policy frameworks. David also works on Forest-Water Dynamics and how these impact moisture vapor transport and water availability across terrestrial space and the potential role of forests in Climate Change mitigation and adaptation.
Matthew McCartney is Theme Leader of Ecosystem Services at the International Water Management Institute (IWMI), specializing in water resources and wetland and hydro-ecological studies.
Jamie Pittock is Associate Professor at the Fenner School of Environment and Society at the Australian National University. He teaches courses on environment and society and climate change vulnerability and adaptation. His research focusses on environmental governance, climate change adaptation, energy and sustainable management of water. Jamie manages major research projects on irrigation and water in Africa and on energy and food in the Mekong region. Since 2010 Jamie has also been Director of International Programs for the UNESCO Chair in Water Economics and Transboundary Water Governance.
Brendan Smith is the Land Use Technical Director at The Gold Standard Foundation based in Geneva. A former ecologist with AECOM in Australia, he successfully led the launch of the Water Benefit Standard for the Gold Standard Foundation in 2014 – one of the first ever global results based approaches to the financing of water projects based upon the benefits they generate.
The views expressed in this article belong to the individual authors and do not represent the views of the membership of IUCN, or constitute formal policy of IUCN, IWMI, GSF, the Global Water Forum, the UNESCO Chair in Water Economics and Transboundary Water Governance, UNESCO, the Australian National University, or any of the institutions to which the authors are associated. Please see the Global Water Forum terms and conditions here